Ca II K line profile of the truly quiet Sun

While evaluating the chromospheric variability (solar cycle related or any other) using the Ca II K line (λ3933.684 A) as an indicator, an essential prerequisite is the knowledge of the profile of a truly quiet Sun in the integrated light. Such a profile can serve as a bench mark over which enhancements can be measured, particularly when modelling variability. This paper describes how such a K-line profile has been derived for the quiet Sun using disc-integrated light.